In certain hydraulic drilling apparatus, in particular rotary/percussion apparatus, the "percussion" function is initiated by the propulsion pressure. Thus the pressure at which the jack for the propulsion of the drill is supplied with hydraulic fluid serves to control the striking of a hammer in the form of a piston, for example by providing that:
as long as said pressure has not reached a certain threshold, the rate of flow of hydraulic fluid controlling the percussion movement is nil;
as soon as this pressure threshold is reached, the rate of flow of hydraulic fluid controlling the percussion movement is brought to a value facilitating "low percussion";
beyond this pressure threshold, the rate of flow increases progressively and in proportion to the pressure in order to achieve operating conditions of "normal percussion".
This self-regulation makes it possible to pass from operating condtions of "low percussion" for starting a hole, to operating conditions of so-called "normal percussion" for normal drilling. However, when at the end of drilling the hole, the propulsion jack approaches the end of its stroke, the system alone is unable to prevent an idle percussion stroke which is useless, leads to deterioration of the apparatus, in particular damage to the tip of the drill, causes heating of the hydraulic fluid and is noisy. In this respect, it should be noted that the association of a simple pressure limiter with the propulsion jack is not sufficient to eliminate these drawbacks, since a device of that type is not able to cancel out the propulsion pressure.